Human and nonhuman primate embryonic stem cells (ESC) have the potential to provide unlimited supplies of healthy cells for the implementation of regenerative medicine and the reconstitution of diseased organs. Yet, equally significant is their potential for providing insights into fundamental principles and mechanisms of early human development. One of the greatest gaps in our knowledge is at the periimplantation stage of human embryonic development, due to limitations on the experimental use of human embryos. We have developed a paradigm with human ESC-derived embryoid bodies which promotes the initiation of trophoblast differentiation, and enhances their differentiated function by culture in extracellular matrix environments. Yet, limitations remain in the use of human cells for in vivo experiments. We thus propose to adapt this in vitro paradigm to rhesus monkey ESC (rESC) and expand it to an in vivo implantation model with rESC-derived embryoid bodies with 2 specific aims: Specific Aim 1. To define trophoblastformation and differentiation (including MHC class I expression) in rESC in response to soluble factors, extracellular matrix environments, and transcriptional intervention. Specific Aim 2. To perform in vivo transfer of rhesus embryoid bodies to evaluate trophoblast differentiation and endometrial responses at a eutopic uterine site. Few models exist for research specifically relevant to human embryonic development and implantation. The embryoid body (EB) model shows substantial promise in allowing the study of human placental morphogenesis, based on functional and morphological correlates of trophoblast function. Our proposed studies will establish a nonhuman primate resource to model placental development and morphogenesis. In addition, the definition of the MHC expression within rhesus ES cells and their derivatives has significance for transplantation biology and immunology as well as implantation biology and development of the extraembryonic fetal membranes.